Yesterday I left my furnace off while I was at work. When I got home it was 48 degrees in the house. I usually keep it at 64 when I'm home so that's what I turned it to when I got home from work. It normally will heat up fairly quickly but it it was slow to warm up and never got above 60. I thought I had a door open or something. Then I noticed the air was lukewarm coming out of the vents. It is normally very warm. I hoped I don't need a new furnace! By the way, I know I need to replace my filter.